1. Field of the Invention
This invention relates to hydrotreating of liquid hydrocarbons. More particularly, this invention relates to a method of reducing the sulfur content of high sulfur feedstocks and to saturating the aromatic content of the feedstock. Hydrodesulfurization of hydrocarbon feedstocks is a well-known process in the petroleum refining art, as is hydrogenation of aromatic hydrocarbons. Hydrocarbon feedstocks such as those commonly referred to as distillates, as well as certain kerosenes and jet fuels, frequently are subjected to hydrotreating to meet specifications as to sulfur content and aromaticity. Both hydrodesulfurization and aromatic saturaton have been carried out extensively in the past. Both processes are part of the broader technology involving hydrotreating of hydrocarbon feedstocks, and generally involve passing the feedstock over a fixed catalyst bed at elevated temperature and pressure.
2. Description of the Prior Art
As mentioned above, both hydrodesulfurization and aromatic saturation have been carried out in the past by passing hydrocarbon feedstocks over fixed catalyst beds at elevated temperature and pressure. In cases where the feedstock has a high sulfur content and a high aromatic content, it is often desirable to both hydrodesulfurize and saturate the feedstock. However, the art has been faced with a catalyst selection dilemma when saturating high sulfur aromatic feedstocks. Catalysts having the best aromatic saturation activity are also the most sensitive to the feedstock sulfur content. That is, the catalyst having high aromatic saturation activity is quickly poisoned by the sulfur in a high sulfur feedstock, such that it is desirable to reduce the sulfur content prior to saturating the aromatic components in the feedstock. This has required, in some cases, separate operations for sulfur reduction and saturation.
It might be possible to develop a catalyst having high aromatic saturation activity which is not poisoned by sulfur, but the art has developed standard catalysts for both sulfur reduction and aromatic saturation which are reliable, economical and long-lived. Accordingly, there is no particular incentive for the art to develop a new catalyst.
The term "hydrotreating" as used herein is intended to include both hydrodesulfurization and aromatic saturation, but is not intended to include the process generally referred to in the art as hydrocracking, which involves more severe conditions than are required for hydrodesulfurization and aromatic saturation.
Several processes have been utilized in the past in an effort to desulfurize and saturate high sulfur aromatic hydrocarbon feedstocks. One such process is described in U.S. Pat. No. 3,592,758 which utilizes a dual catalyst hydrogenation system. The process described therein includes subjecting the feedstock plus hydrogen to catalytic hydrofining followed by subjecting the effluent to catalytic hydrogenation using a second sulfur-sensitive catalyst. The process is described as a single stage process, and includes flowing feedstock and hydrogen serially over both catalyst beds using a cocurrent flow of hydrogen and feedstock over both beds.
A process of desulfurizing and saturating a high sulfur aromatic feedstock using a first stage to desulfurize and a second stage to hydrogenate the feedstock is described in U.S. Pat. No. 3,654,139. Still another related process is described in U.S. Pat. No. 3,673,078 which describes a hydrogenation process using a sulfur-resistant catalyst for the desulfurizing stage and a more active catalyst for the aromatic saturation stage. The process described therein involves passing hydrogen counter-currently to the hydrocarbon feedstock through a single reactor containing the two catalyst beds in series. Each of the above-described prior art processes has its relative advantages and disadvantages, and there has been a continuing need for an improved process of desulfurizing and saturating high sulfur aromatic feedstocks.